chapter 2, section 6 newton’s third law: run and jump
TRANSCRIPT
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Chapter 2, Section 6
Newton’s Third Law: Run and Jump
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December 1, 2011HW: PTG 1-7 Due Tuesday
• Do Now:– WDYS/WDYT
• Pg. 198
• Agenda:– Do Now– LO/SC– Investigate (Demos)
• Learning Objective– Explain the forces
acting on 2 objects that are interacting
• Success Criteria– Provide evidence that
forces come in pairs, with each force acting on a different object
– Use Newton’s 3rd Law to analyze situations
– Describe how Newton’s 3rd Law explains much of the motion you see in your everyday life
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Materials
• Read through the lab. What materials will you need for this investigate?
• 7 minutes
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Part A: Push, Push Back and Pull, Pull Back• Investigate #1:
– When is your motion accelerated?– When is your motion a constant speed?– Thinking about F=ma, what is the source
of the force that causes the acceleration?• Identify the object that pushes on your mass
to cause acceleration
– On what object do you push? In what direction?
– How do the forces compare? – 8 minutes
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Part A: Push, Push Back and Pull, Pull Back• Investigate #2
– Describe the motion of student A– What force caused the motion of student
A?– Describe the motion of student B– What force caused the motion of Student
B?– 5 minutes
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Part A: Push, Push Back and Pull, Pull Back• Investigate #3
– Think about the forces involved when you walk or run on a horizontal surface• Where does the forward force come from?• Compare the amount of the forward force to
the backward force exerted by your shoe with each step.
– Would it be possible to start walking or running on an extremely slippery surface? Explain why/why not in terms of force.
– 5 minutes
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Part A: Push, Push Back and Pull, Pull Back• Investigate #4
– Read and complete– Record the results in your log– Draw a diagram showing the forces
exerted by you and your partner
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Exit Slip• On a half sheet of paper. Answer the
following question
• How many forces act on any given object? Use your lab and provide specific evidence to support your claim.
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• Do Now:– You do not have to
rewrite the LO and SC
– Explain how you can walk on the sidewalk, but it is more difficult to walk on ice
• Agenda:– Do Now– LO/SC– Physics Talk– Quiz
• Learning Objective– Explain the forces
acting on 2 objects that are interacting
• Success Criteria– Provide evidence that
forces come in pairs, with each force acting on a different object
– Use Newton’s 3rd Law to analyze situations
– Describe how Newton’s 3rd Law explains much of the motion you see in your everyday life
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Part B: Observing a Meter Stick Push Back• Investigate #1:
– Draw the free body diagrams in you notebook
• #2 Set up meter stick and books• #3 Place coin on meter stick
– Record what happens using a free-body diagram
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Part B: Observing a Meter Stick Push Back• Investigate #4
– Data Table– How does the deflection of the meter
stick compare to the weight it is supporting. Make a graph to show this relationship
– Is the meter stick deflecting with the coin? Explain
– Use a free body diagram to show the forces acting on 100g mass when it is at rest on the meter stick.
– 8 minutes
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Exit ticket (Success Criteria)• Based on your observations from
today and yesterday, explain Newton’s 3rd Law:– Use examples to demonstrate that forces
come in pairs and act equally and opposite directions.
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December 5, 2011
• Do Now:– You do not have to
rewrite the LO and SC
– Using Newton’s 3rd Law, explain why some objects don’t move while others accelerate?
• Agenda:– Do Now– LO/SC– Physics Talk– Quiz
• Learning Objective– Explain the forces
acting on 2 objects that are interacting
• Success Criteria– Provide evidence that
forces come in pairs, with each force acting on a different object
– Use Newton’s 3rd Law to analyze situations
– Describe how Newton’s 3rd Law explains much of the motion you see in your everyday life
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Physics Talk 2.6• What always
accompanies acceleration?
• What was the force that pushed the person away from the wall in the chair?
• An unbalanced force
• The wall exerted a force on the person that caused her to accelerate away from the wall.
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Physics Talk 2.6• What were the
forces involved when the students pushed each other on the skateboards?
• What was the forces that moves you forward when you walk across a room?
• Each student exerted an equal force against one another.
• Your foot applies a force to the ground and the ground exerts an equal force on you
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Newton’s Third Law• What is Newton’s
Third Law?• For every applied
force, there is an equal and opposite force. The two forces always act on different objects.
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Newton’s Third Law• What are some
examples of Newton’s Third Law?
• The person pushed (applied a force) on the wall and the wall pushes the person
• Student A on the skateboard pushed on student B and student B pushed on student A
• You push on the floor backwards and the floor pushes you forward.
• You pulled on the spring scale. The spring scale pulled on you. These forces were always equal and opposite in direction.
• You cannot touch someone without them touching you back.
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Newton’s Third Law• How does the
ground provide force if it doesn’t look like it’s moving?
• Example
• The floor actually bends a little bit. If you stand in the center of a trampoline, the bend is quite noticeable; however, floors made of wood or concrete provide less of a bend.
• This is like when the meter stick bent under the force of the weight.
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Newton’s Third Law• Equivalent
expressions of Newton’s Third Law
• For every force applied to object A by another object B, there is an equal and opposite for applied to object B by object A
• If you push or pull something, that something pushes or pulls back on you with an equal amount of force in the opposite direction ALWAYS
• Forces always come in pairs
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Free Body Diagrams• How do you draw a
free-body diagram?• Each force is
represented by an arrow.
• The direction of the force matched the direction of the arrow.
• Each arrow labels the kind of force.
• Objects are often drawn as boxes
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Free Body Diagrams• How do you draw
the force of weight?
• Center of mass
• How do you draw other forces?
• The arrow emerges from the object’s center of mass.
• The point at which all the mass of an object is considered to be concentrated.
• Arrows are drawn by arrows emerging from the point of contact
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Newton’s law in the meter stick bend
• What is the first pair of forces on the meter stick?
• What is the second pair of forces acting on the mass?
• The meter stick pushes up on the mass and the mass pushes down on the meter stick.
• Earth pulls down on the mass with a force of gravity and the mass pulls up on the Earth with an equal force of gravity.
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Challenging Newton’s Third Law
• Scenario: If I pull on the chair then the chair pulls on me with an equal force. Therefore, the two forces cancel and nothing should move. Newton’s law must be wrong.
• True or False?
• False! There are additional forces at work.
• The force on the ground is larger than the force on the chair, which makes the person move forward.
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What do you think now?• Locate each success criteria and
complete summaries for each.